Pulmonary devices, such as spirometers, peak flow meters, endotracheal tubes, connectors and the like are used in the testing and treatment of patients with pulmonary problems. A number of such devices, such as endotracheal tubes and connectors, are also used in the treatment of critically ill patients where any increase in the amount of work in breathing should be avoided.
Nevertheless, it has now been found that certain apparatus that is currently available have deficiencies that may have life-threatening consequences. For example, some of the materials selected for use in manufacturing endotracheal tubes are not heat stable in their physical characteristics at body temperatures and do not remain firm enough at those temperatures to retain their desired shapes while being inserted and while in place. Sometimes, a tube will collapse and/or kink, causing irritation if left in place and, more importantly, significantly reducing the rate of flow of air, oxygen, or mixture that can flow through the tube. The total volume of fluid flow per unit of time through a tube is given by Poiseuille's law as follows: ##EQU1## where V=volume of flow
R=radius of the tube PA1 P.sub.1 and P.sub.2 =are the pressures at the respective ends of the tube PA1 n=viscosity of the flowing fluid PA1 L=the length of the tube
From this equation, it is seen that any slight restriction in the radius R of the tube can have a significant reduction in the rate of flow through the tube since the radius is raised to the fourth power.
Similar problems exist in connection with endotracheal tube connectors for connecting an endotracheal tube to a source of oxygen or other gas. There are numerous such connectors on the market which do not minimize the work of breathing by reducing the negative pressure required to inhale a given volume of oxygen-enriched gas.
Accordingly, it is presently believed that there is a significant demand for a test apparatus for testing and evaluating pulmonary devices. Such apparatus would, for example, permit a hospital or doctor to readily determine which endotracheal tube and/or connector is optimal for a patient or patients. The physician or hospital could also screen devices, select the best product for their patients and assure themselves of accurate and repeatable tests and of a device that will not fail under in-use conditions.
In addition, it is presently believed that there is a demand for test apparatus which will measure flow resistance under simulated conditions with the direction of flow in two opposite directions, i.e., corresponding to the inhalation and exhalation of a patient. It is further believed that there is a demand for test apparatus which will produce or generate a particular pulmonary waveform such as any of 24 American Thoracic Society (ATS) Standard Pulmonary Waveforms.
It is also believed that an apparatus for testing pulmonary devices in accordance with the present invention will satisfy the aforementioned demands. Such apparatus are also relatively inexpensive to manufacture, durable, accurate, produce repeatable results and are easy to use.